As CMOS technologies continue to migrate into deep submicron region, the power supply voltage will likewise scale to below 1.5 V for reliable operation of devices. In various hand-held and/or wireless devices it is advantageous for the supply voltage to be reduced even further to keep power consumption and weight low. As an essential and integral part of more and more very large scale integration circuit systems, a temperature-compensated (or commonly called bandgap) reference circuit that works with supply voltages below 1.5 V is desired.
FIG. 1 shows a simplified diagram of a conventional CMOS bandgap reference circuit. The closed loop of on operational amplifier A0 forces the voltages at nodes PT and Q2E to be equal, resulting in a bandgap reference voltage             V      REF        =                                        R            0                                R            PT                          ⁢                  ln          ⁡                      (                                          a                E                            ⁢                              m                2                                      )                          ⁢                  V          T                    +              V        EB2              ,
where aE is the ratio of emitter areas of Q1 over Q2, and M2 is the current ratio, I2/I1. Vxcfx84=kT/q, the thermal voltage, has a positive temperature coefficient and VEB has a negative temperature coefficient of about xe2x88x922MV/xc2x0C. Satisfying the condition dVREF/dT=0 for T=T0 usually results in VRFF≈1.2 V with aE=8, M2 =1. Allowing some voltage drop across the current sources M1 and M2, the minimum supply voltage will typically be VDDxe2x89xa71.5 V.
The minimum supply voltage required to properly operate this circuit is VDDxe2x89xa7VREF+VSD since VREF greater than VEB2. A common technique to lower the minimum VDD is to generate a Proportional To Absolute Temperature (xe2x80x9cPTATxe2x80x9d) current and a current proportional to VEB, and then sum the two currents into a resistor to generate a bandgap voltage that may contain only a fraction of a VEB instead of a whole VEB voltage. This is commonly referred as a fractional VEB bandgap reference.
Bandgap a reference circuits with minimum supply voltages of VDDxe2x89xa70.9 V have been achieved. A first technique results in a bandgap reference voltage VREF greater than VEB, which limits the supply voltage to VDDxe2x89xa70.9 V. A second technique predicted a lowering of supply voltage to VDDxe2x89xa70.85 V, but achieves only VDDxe2x89xa72.1 V due to technology limitations. The second technique requires that two resistors be connected across the emitter-base terminals of two separate PNP transistors to generate a whole VEB current and sum it with a PTAT current. It then forces the resultant current through a third resistor to produce an appropriate bandgap reference voltage. For a given voltage drop, V0, across a resistor having a current, I0, flowing through, the resistance of the resistor is R0=VEB/I0. Therefore, the total resistance of the two resistors connected across the emitter-base terminals of two separate PNP transistors is             R      t        =          2      ⁢                        V          EB                          I          0                      ,
whee I0 is the current flowing through each resistor. For example, I0=1 xcexcA (10xe2x88x926A) and VEB=0.7 V results in R1=1,400,000xcexa9. In integrated circuit technologies, chip area needed to implement a resistor is directly proportional to the total resistance of the resistor. Therefore, additional resistors or resistances requires additional chip area.
Embodiments of the invention provide a bandgap reference circuit that may use reduced substrate area compared to prior art bandgap reference circuits, while requiring relatively low voltage. A first embodiment of the invention includes a bipolar transistor with a resistor electrically connected across the emitter-base of the bipolar transistor. The resistor sums a first current with a second current and also generates a fractional VEB.
In an illustrative embodiment of the invention the bandgap reference circuit has a first current is proportional to VEB, and a second current proportional to a PTAT current.
In a further embodiment of the invention the bandgap reference circuit has an impedance booster.
The present invention also includes a method of regulating a voltage level using embodiments of the bandgap reference circuit.